There are plenty tutorials and videos in Youtube
How do I export my unsaved WhatsApp contacts to excel?
Choose a WhatsApp backup and click Export to Computer. Step 3: Choose your contact files and where you want them to export. Save them in Excel format as XLS. Then click the Export button and your contacts will instantly be formatted into XLS and can be seen in Excel
Get the Chrome Extension [“Download WhatsApp Group Phone Number”
You can just visit the Chrome store to look for any suitable extension to download WhatsApp contacts. One of them is “Download WhatsApp Group Phone Number” that you can find and add to Chrome for free.
After installing the Chrome extension, you have to restart your browser to find its icon on the top-right corner.
Once the Chrome extension is added, you can open WhatsApp on the system via the WhatsApp web feature. After loading WhatsApp on your Chrome, just select the group from where you wish to download the contacts. Tap on the three-dot icon from the top and select the “Download Info” button.
The Chrome extension will automatically download the contacts from the WhatsApp group in the form of a CSV file. Once the file is saved, you can just open it in Excel to extract WhatsApp group contacts.